The present invention relates to spray guns for spraying coating materials such as paint or the like. More particularly, the invention relates to an air spray gun operable under extremely low air pressure conditions.
The use of pressurized air in connection with a spray gun for assisting in the atomization and delivery of sprayed liquid particles is well known. Prior art air spray guns have utilized air under pressure, both for atomizing liquid delivered to the spray gun into fine particles, and also for shaping the cloud of particles emanating from the spray gun. Such spray guns typically utilize air pressures ranging up to approximately 100 pounds per square inch (psi). Some of the problems in utilizing air pressures in this range include the problem of overspray, wherein a very large cloud of liquid particles are emitted at significant velocities, tending to create overspray; i.e., spray which passes by the article to be coated and becomes a source of pollution in the atmosphere. Another problem in utilizing higher air pressures is in the need to deliver significant volumes of air under significant pressures, thereby requiring a considerable amount of energy for producing the requisite pressurized air conditions.
Fairly recent prior art has addressed these problems and, as a result thereof, spray guns have been developed which provide atomizing air at pressures of approximately 10 psi, at relatively high flow rates; i.e., in the range of 18-25 cubic feet per minute (cfm). Spray guns of this general type are known as "high volume low pressure" (HVLP) spray guns, and they provide a higher transfer efficiency in the application of liquid coating materials to articles. They utilize a higher pressure air inlet to the spray gun, and an adjustable pressure restrictor in the air flow passage internal to the spray gun. The air passages downstream from the pressure restrictor are divided into air atomizing passages and fan air passages. The air atomizing passages convey the pressurized air to the liquid orifice outlet, wherein liquid emitted through the outlet becomes atomized under the influence of the pressurized air. The fan air passages are directed to various openings in the spray nozzle, surrounding the atomizing orifice, positioned so as to influence the shape of the atomized cloud of particles which are emitted from the orifice. In a typical spray gun of this type both the air atomizing passages and the fan air passages receive air from the same adjustable valve, and the air pressures are substantially the same through both sets of passages. One example of a spray gun of this type is shown in U.S. Pat. No. 5,064,119, issued Nov. 12, 1991.
A problem with this general type of spray gun is that the adjustment feature affects both the air atomizing passages and the fan air passages simultaneously; when the atomizing air pressure is increased or reduced a corresponding change of the fan air pressure is also produced. This consequence is not always desirable, for in many cases it is desirable to atomize at a very low air pressure, while maintaining a relatively higher air pressure for shaping the air spray fan. This limitation frequently restricts the usefulness of such spray guns. In the case of spraying liquids which are difficult to atomize, it is sometimes necessary to increase the total air flow to a point where overspray is excessive and air energy is wasted, thereby defeating the purpose of HVLP spray guns for gaining higher transfer efficiencies. This condition is difficult to accommodate with existing spray guns, unless two different pressure sources are connected to the spray guns, each source being independently controllable to provide the optimum pressure and flow rate for the respective tasks of controlling atomization and controlling pattern fan size and shape.